Touch panels have become widely adopted as the input device for a range of electronic products such as smartphones and tablet devices.
Most high-end portable and handheld electronic devices now include touch panels. These are most often used as part of a touchscreen, i.e., a display and a touch panel that are aligned so that the touch zones of the touch panel correspond with display zones of the display.
The most common user interface for electronic devices with touchscreens is an image on the display, the image having points that appear interactive. More particularly, the device may display a picture of a button, and the user can then interact with the device by touching, pressing or swiping the button with their finger or with a stylus. For example, the user can “press” the button and the touch panel detects the touch (or touches). In response to the detected touch or touches, the electronic device carries out some appropriate function. For example, the electronic device might turn itself off, execute an application, or the like.
Although a number of different technologies can be used to create touch panels, capacitive systems have proven to be the most popular due to their accuracy, durability and ability to detect touch input events with little or no activation force.
A well-known approach to capacitive sensing applied to touch panels is the projected capacitive approach. This approach includes the mutual-capacitance method and the self-capacitance method.
In the mutual-capacitance method, as shown in FIG. 1, a drive electrode 100 and sense electrode 101 are formed on a transparent substrate (not shown). A changing voltage or excitation signal is applied to the drive electrode 100 from a voltage source 102. A signal is then generated on the adjacent sense electrode 101 by means of capacitive coupling via the mutual coupling capacitor 103 formed between the drive electrode 100 and sense electrode 101. A current measurement unit or means 104 is connected to the sense electrode 101 and provides a measurement of the size of the mutual coupling capacitor 103. When the input object 105 (such as a finger or stylus) is brought into close proximity to both electrodes, it forms a first dynamic capacitor to the drive electrode 106 and a second dynamic capacitor to the sense electrode 107.
If the input object is connected to ground, as is the case for example for a human finger connected to a human body, the effect of these dynamically formed capacitances is manifested as a reduction of the amount of capacitive coupling between the drive and sense electrodes and hence a reduction in the magnitude of the signal measured by the current measurement unit or means 104 attached to the sense electrode 101.
In the self-capacitance method, as shown in FIG. 2, a drive electrode 200 is formed on a transparent substrate (not shown). A changing voltage or excitation signal is applied to the drive electrode 200 from a voltage source 201. A current measurement means 202 is connected to the electrode 200 and provides a measurement of the size of the self-capacitance 203 of the electrode to ground. When the input object 105 is brought into close proximity to the electrode, it changes the value of the self-capacitance 203. If the input object is connected to ground, as is the case for example of a human finger connected to a human body, the effect is to increase the self-capacitance of the electrode to ground 203 and hence to increase the magnitude of the signal measured by the current measurement means 202 attached to the sense electrode 200.
As is well-known and disclosed, for example, in U.S. Pat. No. 5,841,078 (Bisset et al, issued Oct. 30, 1996), by arranging a plurality of drive and sense electrodes in a grid pattern to form an electrode array, the mutual-capacitance sensing method may be used to form a touch panel device. FIG. 3 shows a suitable pattern of horizontal electrodes 300 that may be configured as drive electrodes, and vertical electrodes 301 that may be configured as sense electrodes. An advantage of the mutual-capacitance sensing method is that multiple simultaneous touch input events may be detected.
It is also well-known and disclosed, for example, in U.S. Pat. No. 9,250,735 (Kim et al, issued Feb. 2, 2016), that by arranging a plurality of electrodes in a two dimensional array, and by providing an electrical connection from each electrode to a controller, the self-capacitance sensing method may be used to form a touch panel device that is able to reliably detect simultaneous touches from multiple objects.
FIG. 4 shows one example of such a two dimensional electrode array forming a touch sensor panel. This array includes twelve square electrodes 400 formed on a first layer, with four electrodes arranged in a first direction and three electrodes arranged in a second direction. Vias 401 connect each electrode 400 on the first layer to connecting lines 402 on a second layer. By this means, each electrode 400 is separately connected to a controller by connecting lines 402. The first column of electrodes is connected by connecting lines 404, the second column is connected by connecting lines 405, and the third column is connected by means of connecting lines 406.
In many touch screens the touch panel is a device independent of the display, known as an “out-cell” touch panel. The touch panel is positioned on top of the display, and the light generated by the display crosses the touch panel, with an amount of light being absorbed by the touch panel. In more recent implementations, part of the touch panel is integrated within the display stack, and touch panel and display may share the use of certain structures, such as transparent electrodes. This is known as an “in-cell” touch panel. This integration of the touch panel into the display structure seeks to reduce cost by simplifying manufacture, as well as reducing the loss of light throughput that occurs when the touch panel is independent of the display and located on top of the display stack.
These two approaches are illustrated in FIG. 5A and FIG. 5B. FIG. 5A shows a schematic view of a cross section 500 of an example out-cell touch screen, i.e. a combination of display and touch panel. The touch panel 501 and display 502 are physically separated, and typically the touch panel may be located below the cover glass, although the order and arrangement of the layers may be different. Touch panel controller 503 and display driver 504 control the touch panel and display functionalities respectively, and they are both controlled by the panel processor 505. Alternatively, as shown in cross section 506 of FIG. 5B, the display and touch sensor may be integrated in the same layer 507, which is sandwiched between the other display layers. This is an in-cell touch panel.
GB2542854A (Brown et al, published Apr. 5, 2017) discloses a type of in-cell touch panel that uses the VCOM layer of the display to form touch panel electrodes, which are connected to driving and sensing circuits by an active matrix of TFTs. This structure achieves the benefits of an in-cell touch panel, particularly lower cost and thickness. Compared with a conventional in-cell touch panel, it generally requires fewer connections between the panel and the controller. The electrode size and shape is also reconfigurable, and it can be used with mutual-capacitance and self-capacitance sensing.
FIG. 6 shows an embodiment described in GB2542854A. Two touch unit cells 600 and 601 influence each other through their mutual capacitance 605. These touch unit cells are also influenced by the presence of an object, in this case a human finger 606, through the capacitances 603 and 604 respectively.
On touch unit cell 600, capacitances 603 and 605 are connected at the common node 650, which is a conductive element joining the conductive plates of capacitances 603 and 605 to the active matrix circuit. Electronic switches (for example, transistors such as TFTs in FIGS. 6) 620 and 621 are used to select which of the data lines 640 and 641 are connected to common node 650. This selection depends on the voltage present at the gate nodes 630 and 631, as controlled by respective control lines 610, 611. When gate 630 is in a high state, transistor 620 connects data line 641 with common node 650. When gate 631 is in a high state, transistor 621 connects data line 640 with common node 650.
On touch unit cell 601, capacitances 604 and 605 are connected at the common node 651, which is a conductive element joining the conductive plates of capacitances 605 and 604 to the active matrix circuit. Transistors 622 and 623 are used to select which of the data lines 642 and 643 are connected to common node 651. This selection depends on the voltage present at the gate nodes 632 and 633, as controlled by respective control lines 612, 613. When gate 632 is in a high state, transistor 622 connects data line 643 with common node 651. When gate 633 is in a high state, transistor 623 connects data line 642 with common node 651. Transistor gates 630, 631, 632 and 633 are actuated by means of control lines 610, 611, 612 and 613 respectively.
As will be understood, each touch unit cell has two control lines and two data lines. Touch unit cell 600 has two control lines 610, 611 and two data lines 640, 641, whereas touch unit cell 601 has two control lines 612, 613 and two data lines 642, 643. In this embodiment the two control lines and one data line 640 (or 642) extend generally along the row direction and the other data line 641 (or 643) extends generally along the column direction, but the invention of GB2542854A is not limited to this configuration for the control lines and data lines.
The control lines 610, 611 (612, 613) of FIG. 6 and the data lines 640, 641 (642, 643) of FIG. 6 implement touch functionality, and so may be considered as “touch control lines” and “touch data lines” respectively. It should be understood however that referring to, for example, a data line as a “touch data line” does not necessarily mean that that data line implements only touch functionality—in some embodiments of GB2542854A a data line may be used to implement both touch functionality and display functionality, and in principle a control line may be used to implement both touch functionality and display functionality.
The data lines 640, 641 (642,643) are connected to respective drive circuits (not shown) for supplying drive signals to the data lines, or sensing circuits (not shown). In a typical embodiment of GB2542854A the data line 641 (643) that extends generally along the column direction is connected to a sensing circuit, although other embodiments are also possible. The data line 641 may therefore also be considered as a “sensing data line” (or “sensing/drive data line”) and the data line 640 may be considered as a “drive data line”. The drive circuit and the drive/sensing circuits may conveniently be constituted in the touch panel controller.
In some embodiments, the basic unit cell structure of the active matrix touch panel may include more than two TFTs. For example, a third TFT may be used to amplify the sense signal.
FIG. 7 shows one embodiment of an in-cell active matrix touch sensor panel as described in GB2542854A. In this embodiment there are nine touch sensitive electrodes 700. Other embodiments may have a different number of electrodes. Each electrode 700 is connected to two TFTs 701. Each electrode and pair of TFTs may comprise several unit cells (such as those shown in FIG. 6) connected in parallel. The “SEL1” gate control line 702 controls the state of the TFTs connecting electrodes in the first row to the vertical “SEN” touch data lines 711-713. The “SEL2” gate control line 705 controls the state of the TFTs connecting electrodes in the second row to the vertical “SEN” touch data lines 711-713. The “SEL3” gate control line 708 controls the state of the TFTs connecting electrodes in the third row to the vertical “SEN” touch data lines 711-713. The “SELB1” gate control line 703 controls the state of the TFTs connecting electrodes in the first row to the horizontal “FNC1” touch data line 704. The “SELB2” gate control line 706 controls the state of the TFTs connecting electrodes in the second row to the horizontal “FNC2” touch data line 707. The “SELB3” gate control line 709 controls the state of the TFTs connecting electrodes in the third row to the horizontal “FNC3” touch data line 710.
Control signals may be applied to the control lines 702, 703, 705, 706, 708 and 709 to configure the connections between the electrodes 700 and the touch data lines 704, 707, 710, 711, 712 and 713. In this embodiment, the touch data lines 704, 707, 710, 711, 712 and 713 may be connected to a touch panel controller. For example, horizontal touch data lines 704, 707 and 710 may be used to apply drive signals to one or more rows of electrodes, and vertical touch data lines 711, 712 and 713 may be used to sense the charge on one or more electrodes in one or more columns.
However, a limitation of the structure disclosed in GB2542854A is that several connecting lines are required, including control lines and touch data lines. The control lines cannot easily be shared with the display. These extra lines may be routed in the panel bezel, but routing a large number of lines in the bezel is not desirable as it may increase the width of the bezel area. Alternatively the control lines may be routed within the display active area. However, this is not desirable because it reduces the pixel aperture and thus the efficiency and brightness of the display.
The touch data lines may also be separate from the display, in which case the same problems are encountered with routing them in the bezel or the active area. Alternatively, the touch data lines may be shared with the display data lines. However, this may not be possible due to the requirements of the display and the display driving electronics.
WO2017056900A1 (Hamaguchi et al, published Apr. 6, 2017) discloses a method of driving an active matrix touch panel structure, such as that disclosed in GB2542854A, with an orthogonal drive code in order to make self-capacitance measurements with a high signal to noise ratio. For example, drive signals corresponding to different orthogonal drive codes may be applied to the electrodes in each row via the horizontal touch data lines, and the total charge on the electrodes in each column may be sensed by charge amplifiers via the vertical touch data lines. By making a series of measurements and decoding the signal from each of the orthogonal drive codes, the self-capacitance of each electrode may be measured. However, this requires the same control and data lines as GB2542854A.
A limitation of typical capacitance measurement techniques as conventionally applied to touch panels is that they are incapable of detecting input from non-conductive or insulating objects, for example made of wood, plastic or the like.
U.S. Pat. No. 9,105,255 (Brown et al, issued Aug. 11, 2015) discloses a type of mutual-capacitance touch panel that is able to detect non-conductive objects, and to distinguish whether an object is conductive or non-conductive. This is achieved by measuring multiple mutual capacitances formed over different coupling distances. The type of object (conductive or non-conductive) can be determined based on the changes in the multiple mutual capacitances. The multiple mutual capacitances are formed between an array of row and column electrodes.